Using plastics, especially enameled plastics body-parts in automobiles, requires testing them for impact-strength at temperatures between +23.degree. and -30.degree. C. As a rule the testing typically involves a tensile-impact test, e.g., per DIN 53,488, by means of pendulum impactors or hydraulic universal testers, and a penetration test carried out with dropping-bolt testers or the already cited universal tester according to DIN, for example, 53,443. Well-grounded conclusions on the suitability of plastics as automobile body parts are reached with present-day knowledge only by resorting to such instrumented test procedures.
In such an instrumented test procedure, applied to the tensile-impact test per DIN 53,448 and to the penetration test per DIN 53,443, the force generated during sample deformation is measured by electronic force pickups and the output is provided as a force-displacement diagram. This diagram allows determination of the work done on the sample and, thereby, the impact strength thereof. This presumes that the force shown in the diagram corresponds to the force acting on the sample and is free of any superpositions from recoil or oscillatory phenomena that would hamper and perhaps make impossible the needed analysis.
The only practical testers presently available are hydraulic universal testers because they alone allow testing over a comparatively wide range of speeds at constant testing rates. However, as discussed further below, they also have drawbacks.
Any tester wherein a mass is being accelerated, such as a pendulum impact tester or a dropping-bolt tester, entails a narrow range of speeds for its applicability. Technical grounds preclude speeds exceeding 5.5 m/s, while the available kinetic energy will usually be too low at speeds less than 2.5 m/s to destroy the sample. At the very least, the speed drops strongly during sample deformation, hence spuriousness is contained in the result.
German patent No. 35 29 470 describes a new tensile-impact tester which is well suited for such tests. This tester, however, does not permit penetration testing. European patent document No. 0079979 B1 describes a hydraulic loading unit, in particular for catapult-systems.
The operational axis of hydraulic testers, as a rule, is vertical and these testers are used for tensile-impact tests in which a sample is connected by a clamping head at its upper end to a force pickup mounted to an upper crossbeam of the tester. The lower sample end is connected by another clamping means to a drive means loosely entering a sleeve fastened to the upper piston rod connected to a piston in a hydraulic cylinder. Such hydraulic cylinders are designed conventionally with piston rods on both sides. The sleeve forms a lead-in path required by the relatively heavy piston for its acceleration to the pre-set speed. The upper end of the sleeve moves along the drive means and thereby ruptures the sample. The force pickup measures the force function while a displacement pickup at the lower piston rod measures the displacement.
In penetration tests with universal testers, a clamping means is mounted to the upper end of the piston rod wherein the pane-shaped sample is fastened. The penetration unit is then mounted to the upper crossbeam, where this unit under DIN 53,443 forms a 20-mm diameter bolt of which a hemispherical end faces the sample. According to said standard, the force sensor in the penetration unit shall be mounted as close as possible to the hemispherical end. The sample and the penetration unit must be apart by such a distance that the piston with the clamping means can be accelerated to the preset nominal speed. Designs are furthermore known where the penetration unit is mounted on the piston of the hydraulic cylinder and the clamping means is stationary.
The impinging of the sample on the penetration unit entails the drawback of the sample impacting this unit and, as regards the impulsive traction.test, the impact of the drive means on the upper sleeve end when the above-described penetration and impact-strength tests are carried out. Recoil-oscillations are initiated thereby, which are superposed on the force signal, and these aspects hamper and even render impossible its analysis.
The impact-strength tester of the German patent No. 35 29 470 avoids this problem because, according to its teaching, a very lightweight operational piston is connected directly to the sample, and thus recoil-free signals are achieved.
Manufacturers of motor vehicles, plastics and enamels and plastics processors are in need of a testing system allowing the testing of plastics, whether enameled or not, for impact strength, in comparative and reproducible manner, at temperatures between +213.degree. and -30.degree. C. Because the last two cited enterprises frequently are of medium-size, costly testing disproportionate to earnings is ruled out. While such testing nevertheless is necessary, it is important for the economy as a whole to create a testing system which is economical both as regards purchase and operation.
The above-described known universal testers are well beyond the means of such medium-size enterprises both as regards purchase costs and the high requirements of personnel. Moreover the unsatisfactory signal-quality from such apparatus compounds the difficulty. The tensile-impact tester of the German patent No. 35 29 470 can be applied only in a restricted manner, being useful only for tensile-impact tests.